The gunmen are members of the Bodo tribe and belong to a faction of the National Democratic Front of Bodoland, said regional police inspector general L.R. Bishnoi. Bodo tribesmen have long accused Muslims of sneaking into India illegally from Bangladesh and encroaching on their ancestral land in Assam state.

The dead included six women and three children, police said.

The violence comes at a time of heightened security during India's general election, with the voting taking place over six weeks.

Tensions have been high since a Bodo lawmaker in India's Parliament criticized Muslims for not voting for the Bodo candidate, said Lafikul Islam Ahmed, leader of a Muslim youth organization called the All Bodoland Muslim Students' Union.

In 2012, weeks long violence between Bodo people and Muslims killed as many as 100 people in the same area.

Police said the first overnight attack took place in western Assam, when at least eight rebels opened fire on a group of villagers sitting in a courtyard. Four people were killed and two others wounded, Bishnoi said.

The second attack happened in Kokrajhar district when more than 20 armed men, their faces covered with black hoods, broke open the doors of two homes and sprayed them with bullets, killing seven people, witnesses said.

Crying inconsolably, 28-year-old Mohammed Sheikh Ali said his mother, wife and daughter were killed in the attack.

“I will curse myself forever because I failed to save them,” Ali said in a telephone interview from a hospital, where he was waiting for doctors to complete the autopsies on his family. “I am left all alone in this world ... I want justice.”

The National Democratic Front of Bodoland has been fighting for a separate homeland for the region's ethnic Bodo people for decades. The Bodos are an indigenous tribe in Assam, making up 10 percent of the state's 33 million people.